1. Field of the Invention
The field of the invention is industrial controllers and more specifically controllers for injection molding machines.
2. Background Art
In the injection molding process, a plasticized material held in a "barrel" is forced under pressure, typically by means of a ram fitting within the barrel, through a nozzle in one end of the barrel. The plasticized material enters into a mold cavity under pressure where it solidifies into a molded part in conformance with the dimensions of the mold cavity. The part is then ejected from the mold and the process is repeated.
This injection molding process may be broken into four stages: plastication, injection, packing and holding.
In the plastication stage, solid pellets of the molding material are fed into the barrel where they are melted and forced to the front of the barrel by rotation of a screw forming part of the ram. As the molding material is melted by the mechanical action of the screw, the barrel begins to fill moving the screw and ram back from the nozzle. Control of the ram back-pressure may be used to ensure the melted molding material is at proper temperature and free from voids or air pockets.
In the injection stage, the rotation of the screw ceases and the ram is moved toward the nozzle to force the molding material through the nozzle into the mold cavity. The characteristics of the molding material or of the mold may require that certain parts of the mold cavity be filled at different rates. This may be accomplished by varying the speed or pressure of the ram during the injection stage.
In the packing stage, additional molding material is forced into the mold cavity to accommodate shrinkage of the molding material as it cools in the mold cavity.
In the holding stage, pressure is maintained on the molding material to control its density and/or flexibility. At the conclusion of the holding stage, the molded part shrinks away from the mold cavity prior to ejection of the part. Control of the ram pressure during the holding stage also may prevent distortions of or depressions in the part as it cools.
Two principal control techniques, pressure control and velocity control, have been used to improve the quality and consistency of injection molded parts. With pressure control, a pressure transducer is attached to the hydraulic cylinder driving a piston connected to the ram to measure the ram's pressure against the molding material, or a pressure transducer is fitted to the mold cavity to measure the pressure of the molding material against the cavity wall. The pressure during each of the stages described above is controlled and varied either as a function of time or of ram position.
In the velocity control approach, the ram is instrumented to provide data as to ram position and this position information is processed to provide the velocity of the ram. The ram velocity during the injection stage is then controlled and varied as a function of ram position.